In the Middle Ages, many
people lived inside castles
like this one. You can study pictures of different kinds of medieval
castles, and the excellent book on castles by David Macaulay (called
Castle), and then try designing and building your own model castle
(as a group). Check out Kidipede's pages about medieval castles: the Tower
of London, Carcassonne, the Conciergerie, and the Chateau de Vincennes.

First you'll have to decide on a site, then draw out the plan,
taking into account the need for water inside the walls, and all the
different things that made the castles safer to live in, in case they
were attacked by enemies. Be sure to define the scale of your plan.

More great articles:

Once you have a plan, you can begin building your castle. You might
use wooden blocks, or Legos, or sand if you are at the beach, or cardboard.
How can you match the measurements on the plan to the measurements
of your model?